The steel cutting of the rotation symmetrical surfaces, both in a mild or hardened state, employing cutters with a certain geometry, constitutes the actual state of the art. Thereby the finishing, buffing, honing and the like procedures are partly replaced.
The cutting materials to provide a long enough period of service even for cutting hardened workpieces, are nowadays available in different shapes.
An after hardening machining is as a rule required even if the initial machining takes place with a high level of precision, since after hardening there as a rule partly appear considerable distorsions due to the hardening. Thus a process to restore the exact sizes of the workpiece is a must.
The longitudinal turning always creates a twisted surface, namely regardless of the machined material or whether that is hardened or not respectively.
This surface has regular structures (grooves like prominences) corresponding to a thread shaped structure (twisted) which, by the advance of the tool along the rotating workpiece are producing a screw pitch.
This thing is valid to the same extent for the cylindrical, conical or other differently shaped workpiece surfaces. Thus the surfaces of the workpiece to be machined have for example the shape of a thread or parts of it.
The dependency in accordance with the formula describing the thread depth and the thread shaped structure rugosity respectively, is presented in FIG. 3.
Since the cutting feed-motions, in particular in case of the workpieces hardened for an enhanced surface quality—are as a rule very small, this often leads to a relatively slow machining advance and a low cutting output capacity respectively.
While the relatively low cutting output capacity is an important economic drawback, the twisted surfaces are creating technical problems to the packings arranged on them, in particular whether they are shifting with respect to the surfaces, for example in case of a shaft rotating inside a fixed packing. Such packings may be for example the well-known shaft radial packing washers.
By means of the grooves or the thread shaped barbs respectively, such twisted surface carries cooling fluid, fuel, etc. along the surface, about the packing, from one side to the other in the axial direction, thus that the sealing effect of the packing will be significantly decreased. In particular in case of machines which for example out of hygiene or environment protection reasons have to run without leaking, this thing is a problem worthy of being considered.
Supplementary, because of these grooves or thread shaped prominences, the sealing elements arranged on the twisted surfaces will be often deteriorated in time at their lines or contact surfaces, or at least strongly abrasively stressed. By this wear or deterioration, the sealing effect will be likewise often significantly decreased or annulled before long.
As to the actual steel cutting of the rotating workpieces, considering the above-mentioned problems, there arises the following situation:
Whether on rotating the rotation symmetrical surfaces there is attempted the avoiding of the thread shaped appearance by shifting the tool but radially (transverse motion turning) with respect to the workpiece, due to the lack of the axial motion there is thus obtained a twist-free surface. But in case that the cutter is as broad in the axial direction as the total rotation symmetrical surface to be obtained, there are arising, in particular on machining the hardened surfaces, very great transverse forces, and because of the dynamic instability there is a high vibration tendency. These dynamic instabilities and the vibration respectively, are leading quickly to so great surface irregularities that in this case the surface is likewise too irregular for a perfect sealing.
Whether on broader surfaces to be machined the cutter is supplementary shifting along them, this meaning in the axial direction, there naturally appears again a twisted surface, as long as the cutter is not exactly arranged in parallel to the longitudinal direction. But in case that such a parallelism is obtained, the cutter immediately tends to vibrate.
Therefore, according to the actual state of art, is necessary that the twist thus created, in particular the thread like grooves remaining after turning, be sufficiently decreased or entirely eliminated by means of later expensive supplementary procedures to provide thus a perfect sealing effect.
A possibility to avoid the surface twist (turning grooves), could be the rotary broaching procedure, whereupon the broaching cutter will be shifted in a tangential direction about the rotating workpiece. If the individual cutters of the rotary broaching machine are orientated in parallel with respect to the rotation axis of the workpiece, there arises again here the problem of the great forces acting upon the cutters and thereby both a tendency to dynamic instability and vibration.
On the rotary broaching by means of a disk shaped cutter holder having the cutters on the circumference of the cutter holder, on the oblique arrangement of the straight cutters there supplementary arises the problem of obtaining a workpiece with a convex curved surface instead of an exactly cylindrical one.
Further on, often as a supplementary procedure, there is employed the surface buffing. This means that as a rule the workpiece has to be mounted on another type of machine. This means that the cost price per workpiece will soar up because of an extended operating process involving the employing of supplementary machines, leading thereby to a considerable worsened economic efficiency. Further on, according to the actual state of art, on the final machining of the workpiece, if possible, the buffing should be avoided since this is as a rule carried out as a damp procedure and thereby, because of the buffing slime, there arise supplementary environment and salubrity problems, thus worsening the economic efficiency.
Furthermore, even on buffing there are created twisted structures which as a result of the buffing procedure firstly appear on the buffing wheel, to form in the end on the workpiece. On finishing too, whereupon a finishing band or a finishing element is brought into contact with the workpiece, because of the vibrations and the finishing tool supplementary longitudinal shift with respect to the workpiece respectively, there appear twisted surface structures.
Further on, from DE 199 63 897 there is known the workpiece machining by means of an obliquely arranged cutter with respect to the rotation axis of the workpiece, in that this is tangentially shifted along the workpiece. By controlling the lead and/or the oblique arrangement of the cutter there may be minimized both the pitch and the depth of the twist like grooves, while on machining the hardened workpieces the cutting output capacity of the cutter may be optimized.
A drawback of this procedure consists in having the axial length of the surfaces to be machined limited by the possible length of the small plates in the axial direction.